Solid-state structures of phenyleneethynylenes: Comparison of monomers andpolymers

The solid-state structures of a series of 2,5-dialkyl-substituted poly(p-ph enyleneethynylene)s (PPEs 4, R = hexyl, dodecyl, isopentyl, ethylbutyl, eth ylhexyl, H) and of 2,5-didodecyl(1,4-phenylenebutadiynylene) were investiga ted by X-ray powder and electron diffraction. In addition, X-ray single-cry stal structures of the monomeric (1,4-dipropynyl-2,5-dialkyl)benzenes were obtained and compared to the structures of the corresponding polymers. In t he case of the ethylhexyl-substituted PPE, the packing of the monomer resem bles that of the polymer in the solid state. All of the examined PPEs show lamellar morphologies, in which the extended geometry of the side chains de termines the value of the lamellar spacing obtained by powder diffraction. A second d value, which could be obtained for all samples represents the pi -pi-stacking distance of the main chains. This distance is around 3.8 Angst rom, a typical value for stacking of aromatic systems. It implies strong el ectronic interaction of the chromophoric backbones, leading to the observed sharp aggregate band in CHCl3/MeOH solutions and in thin films. From the s cattering data, staggered packing of the benzene rings and the phenyleneeth ynylene main chains in PPEs 4 was inferred. This packing avoids repulsive e lectrostatic interactions between adjacent conjugated phenyleneethynylene c hains and at the same time leads to optimal packing of the side chains.